Polyolefin production

ABSTRACT

Provided is a process for producing a polyolefin having a multimodal molecular weight distribution, which process comprises: (a) polymerizing a first olefin monomer in the presence of an isomerizable metallocene catalyst, to form a first multi-modal polyolefin component; and (b) polymerizing a second olefin monomer in the presence of a second metallocene catalyst to form a second polyolefin component; wherein the molecular weight distribution of the first polyolefin component overlaps with the molecular weight distribution of the second polyolefin component.

The present invention concerns processes for producing polyolefinshaving a controlled multimodal molecular weight distribution. Theinvention also relates to polyolefins produced using the process of theinvention.

In many applications in which polyolefins are employed, it is desirablethat the polyolefin used has good mechanical properties. It is knownthat, in general, high molecular weight polyolefins have good mechanicalproperties. Additionally, since the polyolefin must usually undergo someform of processing (such as moulding processes and extrusion processesand the like) to form the final product, it is also desirable that thepolyolefin used has good processing properties. However, unlike themechanical properties of the polyolefin, its processing properties tendto improve as its molecular weight decreases.

Thus, a problem exists to provide a polyolefin which simultaneouslyexhibits favourable mechanical properties and favourable processingproperties. Attempts have been made in the past to solve this problem,by producing polyolefins having both a high molecular weight component(HMW) and a low molecular weight component (LMW). Such polyolefins haveeither a broad molecular weight distribution (MWD), or a multimodalmolecular weight distribution.

There are several methods for the production of multimodal or broadmolecular weight distribution polyolefins. The individual polyolefinscan be melt blended, or can be formed in separate reactors in series.Use of a dual site catalyst for the production of a bimodal polyolefinresin in a single reactor is also known.

Chromium catalysts for use in polyolefin production tend to broaden themolecular weight distribution and can in some cases produce bimodalmolecular weight distribution, but usually the low molecular part ofthese resins contains a substantial amount of the co-monomer. Whilst abroadened molecular weight distribution provides acceptable processingproperties, a bimodal molecular weight distribution can provideexcellent properties.

Ziegler-Natta catalysts are known to be capable of producing bimodalpolyethylene using two reactors in series. Typically, in a firstreactor, a low molecular weight homopolymer is formed by reactionbetween hydrogen and ethylene in the presence of the Ziegler-Nattacatalyst. It is essential that excess hydrogen be used in this processand, as a result, it is necessary to remove all the hydrogen from thefirst reactor before the products are passed to the second reactor. Inthe second reactor, a copolymer of ethylene and hexene is made so as toproduce a high molecular weight polyethylene.

Metallocene catalysts are also known in the production of polyolefins.For example, EP-A-0619325 describes a process for preparing polyolefinshaving a bimodal molecular weight distribution. In this process, acatalyst system which includes two metallocenes is employed. Themetallocenes used are, for example, a bis(cyclopentadienyl) zirconiumdichloride and an ethylene-bis(indenyl) zirconium dichloride. By usingthe two different metallocene catalysts in the same reactor, a molecularweight distribution is obtained, which is at least bimodal.

A problem with known bimodal polyolefins is that if the individualpolyolefin components are too different in molecular weight and density,they may not be as miscible with each other as desired and harshextrusion conditions or repeated extrusions are necessary which mightlead to partial degradation of the final product and/or additional cost.Thus the optimum mechanical and processing properties are not achievedin the final polyolefin product. Thus, many applications still requireimproved polyolefins and there is still a need to control the molecularweight distribution of the polyolefin products more closely, so that themiscibility of the polyolefin components can be improved, and in turnthe mechanical and processing properties of the polyolefins can befurther improved.

It is therefore an object of the present invention to solve the problemsassociated with the known catalysts, processes and polymers.Accordingly, the present invention provides a process for producing apolyolefin having a multimodal molecular weight distribution, whichprocess comprises:

-   -   (a) polymerising a first olefin monomer in the presence of an        isomerisable metallocene catalyst, to form a first multimodal        polyolefin component; and    -   (b) polymerising a second olefin monomer in the presence of a        second metallocene catalyst to form a second polyolefin        component;        wherein the molecular weight distribution of the first        polyolefin component overlaps with the molecular weight        distribution of the second polyolefin component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A demonstrates the molecular weight of a combination of a catalystsystem producing a polyethylene having a narrow molecular weightdistribution with a catalyst system producing a polyethylene with abimodal molecular weight distribution.

FIG. 1B demonstrates the molecular weight of another combination of acatalyst system producing a polyethylene having a narrow molecularweight distribution with a catalyst system producing a polyethylene witha bimodal molecular weight distribution.

FIG. 1C demonstrates the molecular weight of yet another combination ofa catalyst system producing a polyethylene having a narrow molecularweight distribution with a catalyst system producing a polyethylene witha bimodal molecular weight distribution.

The isomerisable catalyst is a catalyst which is able to form two ormore different isomers, each of which contributes differently to the MWDof the resulting polymer. It is this different contribution which leadsto the multimodal nature of the first polyolefin component. Preferredcatalysts of this type are bisindenyl metallocene catalysts.

In the context of the present invention, a multimodal molecular weightdistribution means a molecular weight distribution which occurs due tothe polyolefin product comprising a mixture of polyolefins havingdifferent molecular weight distributions. Thus, in the presentinvention, a polyolefin may be multimodal even if it has only a singlepeak in its molecular weight distribution, as well as when it has morethan one peak in its molecular weight distribution.

In the context of the present invention, the molecular weightdistribution of one component overlaps with the molecular weightdistribution of a second component if a proportion of the polymermolecules in one component has the same molecular weight as a proportionof polymer molecules in the second component.

An advantage of the present method is that a polyolefin having acontrolled molecular weight distribution can be formed. By tailoring thecatalyst components and reaction conditions to select individualpolyolefin components having a desired MWD, a final product having amore predictable and controlled MWD can be produced. In particular, theformation of the second polymer component having an MWD overlapping withthat of the first component facilitates the mixing of the components andallows a final product to be produced which has improved mechanicalproperties and improved processing properties.

The present invention will now be discussed in more detail. In aparticularly preferred embodiment of the present process, thepolymerising steps (a) and (b) take place in a single reaction zone,under polymerising conditions in which the catalysts producing thepolymer components are simultaneously active. In this embodiment it ispreferred that the metallocene catalysts producing the polymercomponents are part of a multi-component catalyst system, such as atwo-site (dual component) catalyst system. A multi-site catalyst systemis a system in which a plurality of catalysts are present on individualgrains of catalyst support.

In an alternative embodiment, the polymerising steps (a) and (b) maytake place in two or more reaction zones in series, the polymerisingconditions in each reaction zone being selected such that one or more ofthe catalyst components is inactive in each reaction zone. Thus, takingas an example a two-site catalyst, in this embodiment one catalystcomponent may be active in a first reaction zone and a second may beactive in a second reaction zone.

Preferably, the first polyolefin component comprises a bimodalpolyolefin. It is also preferred that the second polyolefin componentcomprises a monomodal polyolefin. However, the modality of the secondcomponent is not limited provided that it's MWD overlaps with that ofthe first component.

The breadth of the MWD can be determined in accordance with any methodgenerally used in the art. Preferably the breadth is determinedaccording to the polydispersion index PDI), usually denoted by a Dvalue. The PDI is defined as M_(w)/M_(n), or the weight averagemolecular weight divided by the number average molecular weight. In thecase of the bimodal component it is preferable that the polydispersionindex, D, is from 5-9, more preferably D=5-7. In the case of the secondcomponent D is preferably less than 3 and more preferably D=2-3.

The molecular weight and proportion of the second polyolefin componentis controlled so that it emphasises either the lower molecular weightfraction of the final polymer, or the higher molecular weight fractionof the final polymer, depending upon the properties that are desired inthe final polymer as exemplified in FIG. 1 that represents threecombinations of a catalyst system producing a polyethylene having anarrow molecular weight distribution with a catalyst system producing apolyethylene with a bimodal molecular weight distribution. As can beseen from that Figure, the resulting molecular weight distribution canbe tailored to produce polymers with the desired properties. Thus, theproportions of each component are not especially limited, provided thatthe desired improved mechanical properties and processing properties areachieved. Preferably the final polymer comprises from 35-49 wt. % of thepolyolefin fraction of higher molecular weight and from 51-65 wt. % of apolyolefin fraction of lower molecular weight. More preferably, thefinal polymer comprises at least 55 wt. % of the fraction of lowermolecular weight, most preferably at least 56 wt. %. The fraction oflower (or higher) molecular weight may be either the first or the secondpolyolefin component, depending on the respective MWDs of eachcomponent.

The resin according to the present invention preferably comprises notmore than 45% by weight of the first polyethylene fraction of highmolecular weight, most preferably at most 44 weight %.

Typically, the isomerisable metallocene catalyst comprises a bisindenylmetallocene catalyst. In a preferred embodiment, bisindenyl metallocenecatalyst has the following formula:(Ind)₂R″MQ_(p);wherein each Ind is the same or different and is a substituted orunsubstituted indenyl group or a substituted or unsubstitutedtetrahydroindenyl group, R″ is a structural bridge impartingstereorigidity to the component; M is a metal atom from group IVB, VB orVIB of the periodic table; p is the valence of M minus 2; and each Q isa hydrocarbon having from 1-20 carbon atoms or is a halogen.

The ligand, Ind, used in catalysts discussed above is an indenyl-typeligand, in which, in the context of the present invention, thesubstituent positions are numbered from 1-7 according to the system setout in the structure below (although an indenyl compound is shown, thenumbering is the same as in the indenyl or tetrahydroindenyl ligand):

To distinguish substitution in the first ligand from the second, thesecond is numbered according to the same system, but from 1′-7′, inaccordance with convention. In this type of catalyst, the position ofthe bridge is not particularly limited, and is preferably a 1,1′-bridge,a 2,2′-bridge or a 1,2′-bridge, a 1,1′-bridge being most preferred.

The substitution pan em of the indenyl groups is not especially limited,provided that the catalyst is isomerisable. Thus, one or both of theindenyl groups may be substituted or unsubstituted. Symmetricalsubstitution patterns are preferred (i.e. both Ind groups aresubstituted in the same positions with the same substituents). Theindenyl groups of the catalyst are preferably substituted at the 2, 2′,4 and/or 4′ positions.

The substituents are not particularly limited and may comprise anyorganic group and/or one or more atoms from any of groups IIIA, IVA, VA,VIA or VIIA of the Periodic Table, such as a B, Si, N, P, O, or S atomor a halogen atom (e.g. F, Cl, Br or I).

When the substituent comprises an organic group, the organic grouppreferably comprises a hydrocarbon group. The hydrocarbon group maycomprise a straight chain, a branched chain or a cyclic group.Independently, the hydrocarbon group may comprise an aliphatic or anaromatic group. Also independently, the hydrocarbon group may comprise asaturated or unsaturated group.

When the hydrocarbon comprises an unsaturated group, it may comprise oneor more alkene functionalities and/or one or more alkynefunctionalities. When the hydrocarbon comprises a straight or branchedchain group, it may comprise one or more primary, secondary and/ortertiary alkyl groups. When the hydrocarbon comprises a cyclic group itmay comprise an aromatic ring, an aliphatic ring, a heterocyclic group,and/or fused ring derivatives of these groups. The cyclic group may thuscomprise a benzene, naphthalene, anthracene, indene, fluorene, pyridine,quinoline, thiophene, benzothiophene, furan, benzofuran, pyrrole,indole, imidazole, thiazole, and/or an oxazole group, as well asregioisomers of the above groups.

The number of carbon atoms in the hydrocarbon group is not especiallylimited, but preferably the hydrocarbon group comprises from 1-40 Catoms. The hydrocarbon group may thus be a lower hydrocarbon (1-6 Catoms) or a higher hydrocarbon (7 C atoms or more, e.g. 7-40 C atoms).The number of atoms in the ring of the cyclic group is not especiallylimited, but preferably the ring of the cyclic group comprises from 3-10atoms, such as 3, 4, 5, 6 or 7 atoms.

The groups comprising heteroatoms described above, as well as any of theother groups defined above, may comprise one or more heteroatoms fromany of groups IIIA, IVA, VA, VIA or VIIA of the Periodic Table, such asa B, Si, N, P, O, or S atom or a halogen atom (e.g. F, Cl, Br or I).Thus the substituent may comprise one or more of any of the commonfunctional groups in organic chemistry, such as hydroxy groups,carboxylic acid groups, ester groups, ether groups, aldehyde groups,ketone groups, amine groups, amide groups, imine groups, thiol groups,thioether groups, sulphate groups, sulphonic acid groups, and phosphategroups etc. The substituent may also comprise derivatives of thesegroups, such as carboxylic acid anhydrydes and carboxylic acid halides.

In addition, any substituent may comprise a combination of two or moreof the substituents and/or functional groups defined above.

Preferably the indenyl groups of the catalyst are substituted by a bulkygroup. In these embodiments, the bulky group is typically selected froma methyl group, an isopropyl group, a tertiary butyl group, atrimethylsilyl group, and a phenyl group. It is particularly preferredthat the bulky group is in the 4-position.

In the more preferred embodiments of the invention, the 2-position and4-position are independently substituted with a methyl, isopropyl,phenyl, ethyl or trifluoromethyl group. The other positions on theindenyl groups are preferably substituted with hydrogen, but may besubstituted with one of the preferred substituents for the 2 and4-positions, if desired. The most preferred system comprises a methylgroup at the 2-position and a phenyl group at the 4-position, withhydrogen groups at the remaining positions.

When there is a phenyl group attached to the indenyl group, it may forma bicyclic system by being attached to two adjacent positions on theindenyl system, to form a benzindenyl system. It is particularlypreferred that the phenyl group is attached to the 4 and 5-positions insuch instances.

Preferably, the bridging group R″ is an alkylidene group or a silylgroup. The alkylidene group is preferably a C₁-C₄ alkylidene group. Itis particularly preferred that the bridging group comprises asubstituted or unsubstituted ethylidene group.

The catalyst for forming the second component is not especially limitedprovided that it is a metallocene and produces a component whose MWD isoverlapping with that of the first component. Typically the catalystcomprises a metallocene having a cyclopentadienyl ligand and a fluorenylligand. In these embodiments it is preferred that the catalyst forforming the second polyolefin component has the following formula:R″(CpR¹R²R³)(Cp′R_(n)′)MQ₂wherein Cp is a substituted or unsubstituted cyclopentadienyl ring; Cp′is a substituted or unsubstituted fluorenyl ring; R″ is a structuralbridge imparting stereorigidity to the component; R¹ is a substituent onthe cyclopentadienyl ring which is distal to the bridge, which distalsubstituent comprises a hydrogen or a bulky group of the formula XR*₃ inwhich X is chosen from Group IVA, and each R* is the same or differentand chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms; R²is a substituent on the cyclopentadienyl ring which is proximal to thebridge and positioned non-vicinal to the distal substituent and is ahydrogen or is of the formula YR#₃ in which Y is chosen from group IVA,and each R# is the same or different and chosen from hydrogen orhydrocarbyl of 1 to 7 carbon atoms, R³ is a substituent on thecyclopentadienyl ring which is proximal to the bridge and is a hydrogenor is of the formula ZR$₃, in which Z is chosen from group IVA, and eachR$ is the same or different and chosen from hydrogen or hydrocarbyl of 1to 7 carbon atoms; n is an integer of from 0-8; each R′_(n) is the sameor different and is a group AR′″3 in which A is chosen from group IVAand each R′″ is the same or different and chosen from hydrogen or ahydrocarbyl having 1 to 20 carbon atoms; M is a Group IVB transitionmetal or vanadium; and each Q is hydrocarbyl having 1 to 20 carbon atomsor is a halogen.

Preferably X, Y, Z and A are independently selected from carbon andsilicon. Typically R¹ is selected from C(CH₃)₃, C(CH₃)₂Ph, CPh₃ andSi(CH₃)₃. Generally R² is CH₃ and R³ is CH₃.

In a preferred embodiment n is 2 and more preferably the fluorenyl groupis substituted at the 3-position and the 6-position. Typically each R′is selected from C(CH₃)₃ and Si(CH₃)₃. Most preferably, the R′ groupsare the same. It is also preferred that the fluorenyl ring isunsubstituted at both positions 4 and 5.

R″ is typically selected from alkylidene having 1-20 carbon atoms, adialkyl germanium or silicon or siloxane, an alkyl phosphine and anamine. More preferably, R″ is isopropylidene or dimethylsilanediyl.

For both the catalysts for steps (a) and (b) M is typically zirconium ortitanium and Q is typically a halogen, such as chlorine.

The present process can be applied to produce any polyolefin, but it ispreferred that the process is employed for producing a polyethylene or apolypropylene.

The present invention also provides a polyolefin obtainable according toa process as defined above and a multisite catalyst system for producinga polyolefin having a multimodal molecular weight distribution, whichcatalyst system comprises two or more catalyst components immobilised ona support, wherein the catalyst components comprise one or more catalystcomponents as defined above.

The polyolefins of the present invention, and in especially thepolyethylenes of the present invention, are particularly advantageouswhen used in pipes and films, and in blow moulding processes.

In addition to the above catalyst components, the catalyst system of thepresent invention may comprise one or more activating agents capable ofactivating any one or more of the catalyst components. Typically, theactivating agent comprises an aluminium- or boron-containing activatingagent.

Suitable aluminium-containing activating agents comprise an alumoxane,an alkyl aluminium compound and/or a Lewis acid.

The alumoxanes that can be used in the present invention are well knownand preferably comprise oligomeric linear and/or cyclic alkyl alumoxanesrepresented by the formula (I):

for oligomeric linear alumoxanes; and formula (II)

for oligomeric cyclic alumoxanes,wherein n is 1-40, preferably 10-20; m is 3-40, preferably 3-20; and Ris a C₁-C₈ alkyl group, preferably methyl. Generally, in the preparationof alumoxanes from, for example, aluminium trimethyl and water, amixture of linear and cyclic compounds is obtained.

Suitable boron-containing activating agents may comprise atriphenylcarbenium boronate, such astetrakis-pentafluorophenyl-borato-triphenylcarbenium as described inEP-A-0427696:

or those of the general formula below, as described in EP-A-0277004(page 6, line 30 to page 7, line 7):

Other preferred activating agents include hydroxy isobutylaluminium anda metal aluminoxinate. These are particularly preferred when at leastone Q in the general formula for metallocenes comprises an alkyl group.

The metallocene catalyst system is generally employed in a slurryprocess, which is heterogeneous.

In the present catalyst system, the catalyst components are immobilisedon an inert support, particularly a porous solid support such as talc,inorganic oxides and resinous support materials such as polyolefin.Preferably, the support material is an inorganic oxide in its finallydivided form.

Suitable inorganic oxide materials which are desirably employed inaccordance with this invention include Group IIA, IIIA, IVA or IVB metaloxides such as silica, alumina and mixtures thereof. Other inorganicoxides that may be employed either alone or in combination with thesilica, or alumina are magnesia, titania, zirconia, and the like. Othersuitable support materials, however, can be employed, for example,finely divided functionalised polyolefins such as finely dividedpolyethylene. Preferably, the support is a silica having a surface areacomprised between 200 and 900 m²/g and a pore volume comprised between0.5 and 4 ml/g.

The amount of alumoxane and metallocenes usefully employed in thepreparation of the solid support catalyst can vary over a wide range.Preferably the aluminium to transition metal mole ratio is in the rangebetween 1:1 and 100:1, preferably in the range between 5:1 and 50:1.

The order of addition of the metallocenes and alumoxane to the supportmaterial can vary. In accordance with a preferred embodiment of thepresent invention alumoxane dissolved in a suitable inert hydrocarbonsolvent is added to the support material slurried in the same or othersuitable hydrocarbon liquid and thereafter a mixture of the metallocenecatalyst component is added to the slurry.

Preferred solvents include mineral oils and the various hydrocarbonswhich are liquid at reaction temperature and which do not react with theindividual ingredients. Illustrative examples of the useful solventsinclude the alkanes such as pentane, iso-pentane, hexane, heptane,octane and nonane; cycloalkanes such as cyclopentane and cyclohexane;and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.

Preferably the support material is slurried in toluene and themetallocene and alumoxane are dissolved in toluene prior to addition tothe support material.

Where the reaction is performed in a slurry using, for example,isobutane, a reaction temperature in the range 70° C. to 110° C. may beused.

In accordance with the invention, the olefin monomer, such as ethyleneand the alpha-olefinic co-monomer is supplied to the reaction zonecontaining the metallocene catalyst. When making a LMW component,typically hydrogen is introduced into the reaction zone. When making aHMW component, typically an α-olefinic co-monomer is added to thereaction zone. Typical co-monomers include hexene, butene, octene ormethylpentene, preferably hexene.

The present invention also provides a polyolefin obtainable according toa process as defined above. The polyolefin of the present invention iseasy to process because of its bimodal character and it can be used indiverse applications such as for examples pipes, films and blow mouldedarticles.

Moreover, the invention also provides a multisite catalyst system forproducing a polyolefin having a multimodal molecular weightdistribution, which catalyst system comprises two or more catalystcomponents immobilised on a support, wherein the catalyst componentscomprise a first catalyst component as defined above in respect of step(a), and a second catalyst component as defined above in respect of step(b).

The present invention further provides use of a multisite catalystsystem as defined above, to produce a polyolefin having a controlledmultimodal molecular weight distribution.

1. A process for producing a polyolefin having a multimodal molecularweight distribution, which process comprises: (a) polymerizing a firstolefin monomer in the presence of a bis-indenyl metallocene catalyst, toform a first multimodal polyolefin component, said bis-indenylmetallocene catalyst being represented by formula (I):(Ind)₂R″MQ_(p)  (I)  wherein: each Ind is the same or different and is asubstituted or unsubstituted indenyl group, or a substituted orunsubstituted tetrahydroindenyl group; R″ is a structural bridgeimparting stereorigidity to the component; M is a metal atom from groupIVB, VB or VIB of the periodic table; p is the valence of M minus 2; andeach Q is a hydrocarbon having from 1-20 carbon atoms or is a halogen;and (b) polymerizing a second olefin monomer in the presence of a secondmetallocene catalyst to form a second polyolefin component, said secondmetallocene catalyst being represented by formula (II):R″(CpR¹R²R³)(Cp′R′_(n))MQ₂  (II)  wherein:  Cp is a substituted orunsubstituted cyclopentadienyl ring; C_(p)′ is a substituted orunsubstituted fluorenyl group; R″ is a structural bridge impartingstereorigidity to the component; R¹ is a substituent on thecyclopentadienyl ring which is distal to the bridge, which distalsubstituent comprises a hydrogen or a bulky group of the formula XR*₃ inwhich X is chosen from Group IVA, and each R* is the same or differentand chosen from hydrogen or hydrocarbyl of from 1-20 carbon atoms; R² isa substituent on the cyclopentadienyl ring which is proximal to thebridge and positioned non-vicinal to the distal substituent and is ahydrogen or is of the formula YR#₃ in which Y is chosen from group IVA,and each R# is the same or different and chosen from hydrogen orhydrocarbyl of 1-7 carbon atoms, R³ is a substituent on thecyclopentadienyl ring which is proximal to the bridge and is a hydrogenor is of the formula ZR$₃, in which Z is chosen from group IVA, and eachR$ is the same or different and chosen from hydrogen or hydrocarbyl of1-7 carbon atoms; n is an integer of from 0-8; each R′ is the same ordifferent and is a group AR′″₃ in which A is chosen from group IVA andeach R′″ is the same or different and chose from hydrogen or ahydrocarbyl having 1-20 carbon atoms; wherein X, Y, Z and A areindependently selected from carbon and silicon; M is a Group IVBtransition metal or vanadium; and each Q is hydrocarbyl having 1-20carbon atoms or is a halogen; and  wherein the molecular weightdistribution of the first polyolefin component overlaps with themolecular weight distribution of the second polyolefin component; and wherein the steps (a) and (b) are carried out in the same reactionzone.
 2. The process of claim 1 wherein the first polyolefin componentcomprises a bimodal polyolefin.
 3. The process of claim 2 wherein thesecond polyolefin component comprises a monomodal polyolefin.
 4. Theprocess of claim 1 wherein the indenyl groups of the catalyst areunsubstituted or are substituted at the at least one of the 2, 2′, 4and/or 4′ positions.
 5. The process of claim 4 wherein the indenylgroups are symmetrically substituted.
 6. The process of claim 4 whereinthe indenyl groups of the catalyst are substituted by a bulky group atat least one of the 4 and 4′ positions.
 7. The process of claim 6wherein said bulky group is selected from the group consisting of amethyl group, an isopropyl group, a tertiary butyl group, atrimethylsilyl group, and a phenyl group.
 8. The process of claim 7wherein said bulky group is a phenyl group which forms a benzindenylgroup with the indenyl group to which it is attached.
 9. The process ofclaim 6 wherein the indenyl groups are substituted at at least one ofthe 2 and 2′ positions by a methyl group.
 10. The process of claim 4wherein the bridging group R″ in the metallocene of formula (I) is aC₁-C₄ alkylene group.
 11. The process of claim 10 wherein said bridginggroup comprises a substituted or unsubstituted ethylene group.
 12. Theprocess of claim 1 wherein in the cyclopentadienyl-fluorenyl group, R¹is selected from the group consisting of C(CH₃)₃, C(CH₃)₂Ph, CPh₃ andSi(CH₃)₃.
 13. The process of claim 12 wherein R² is CH₃.
 14. The processof claim 13 wherein R³ is CH₃.
 15. The process of claim 1 wherein n is2.
 16. The process of claim 15 wherein the fluorenyl group issubstituted at the 3 position and the 6 position.
 17. The process ofclaim 16 wherein each R′ is selected from the group consisting ofC(CH₃)₃ and Si(CH₃)₃.
 18. The process of claim 17 wherein the R′ groupsare the same.
 19. The process of claim 12 wherein R″ in formula (II) isselected from alkylidene having 1-20 carbon atoms, a dialkyl germanium,silicon or siloxane, an alkyl phosphine and an amine.
 20. The process ofclaim 19 wherein R″ in formula (II) is isopropylidene ordimethylsilanediyl.
 21. The process of claim 16 wherein the fluorenylring is unsubstituted at both positions 4 and
 5. 22. The process ofclaim 1 wherein M is zirconium or titanium.
 23. The process of claim 22wherein Q is a halogen.
 24. A process for producing a polyolefin havinga multimodal molecular weight distribution, which process comprises: (a)polymerizing a first olefin monomer in the presence of a bis-indenylmetallocene catalyst, to form a first multimodal polyolefin component,said bis-indenyl metallocene catalyst being represented by formula (I):(Ind)₂R″MQ_(p)  (I)  wherein:  each Ind is the same or different and isa substituted or unsubstituted indenyl group, or a substituted orunsubstituted tetrahydroindenyl group; R″ is a structural bridgeimparting stereorigidity to the component; M is a metal atom from groupIVB, VB or VIB of the periodic table; p is the valence of M minus 2; andeach Q is a hydrocarbon having from 1-20 carbon atoms or is a halogen;and (b) polymerizing a second olefin monomer in the presence of a secondmetallocene catalyst to form a second polyolefin component, said secondmetallocene catalyst being represented by formula II:R″(CpR¹R²R³)(Cp′R′_(n))MQ₂  (II)  wherein: Cp is a substituted orunsubstituted cyclopentadienyl ring; Cp′ is a substituted orunsubstituted fluorenyl group; R″ is a structural bridge impartingstereorigidity to the component; R¹ is a substituent on thecyclopentadienyl ring which is distal to the bridge, which distalsubstituent comprises a hydrogen or a bulky group of the formula XR*₃ inwhich X is chosen from Group IVA, and each R* is the same or differentand chosen from hydrogen or hydrocarbyl of from 1-20 carbon atoms; R² isa substituent on the cyclopentadienyl ring which is proximal to thebridge and positioned non-vicinal to the distal substituent and is ahydrogen or is of the formula YR#₃ in which Y is chosen from group IVA,and each R# is the same or different and chosen from hydrogen orhydrocarbyl of 1-7 carbon atoms, R³ is a substituent on thecyclopentadienyl ring which is proximal to the bridge and is a hydrogenor is of the formula ZR$₃, in which Z is chosen from group IVA, and eachR$ is the same or different and chosen from hydrogen or hydrocarbyl of1-7 carbon atoms; n is an integer of from 0-8; each R′ is the same ordifferent and is a group AR′″₃ in which A is chosen from group IVA andeach R′″ is the same or different and chose from hydrogen or ahydrocarbyl having 1-20 carbon atoms; wherein X, Y, Z and A areindependently selected from carbon and silicon; M is a Group IVBtransition metal or vanadium: and each Q is hydrocarbyl having 1-20carbon atoms or is a halogen; and  wherein the molecular weightdistribution of the first polyolefin component overlaps with themolecular weight distribution of the second polyolefin component; and wherein the polymerization of paragraphs (a) and (b) are carried out inat least two series-connected reaction zones.